JPS5896851A - Ferritic stainless steel for formed steel plate with improved ridging resistance - Google Patents

Abstract

PURPOSE:To obtain a ferritic stainless steel for a formed steel plate reducing remarkably the occurrence of ridges by adding specified amounts of Cr, C and N to a steel. CONSTITUTION:The composition of a stainless steel is composed of, by weight, 15.5-17.5% Cr, 0.01-0.04% C, 0.04-0.08% N and the balance Fe with inevitable impurities. N is effective in improving the metallic structure of the steel, and >= 0.04% N is required, yet>0.08% N deteriorates the formability of a product steel plate. C has an effect of raising the nose temp. of the gamma-phase precipitation curve, yet it is necessary that the C content is restricted to <=0.04% to attain the effect of N. An enormous cost is required in steel manufacture to reduce the C content to<0.01%, so the C content is restricted to 0.01-0.04%.

Description

DETAILED DESCRIPTION OF THE INVENTION Invention 1-XI) This invention relates to a ferritic stainless steel for forming steel sheets in which the occurrence of dinging is significantly reduced.

8US450 is a popular ferritic stainless steel (
430 steel is used for sinks, etc.
It is widely used in ST room equipment and other various products. However, when this type of copper steel sheet is press-formed, uneven surface undulations called ridging occur.

As we all know, this ridging impairs the beauty of the molded product and lowers its value, so polishing must be performed after molding depending on the extent of the ridging, which is a major problem that complicates the process and increases costs. Out. Although much effort has been made to reduce such ridging, no industrially effective preventive measures have yet been found.

In recent years, due to advances in continuous casting technology, a considerable amount of 430 steel production has shifted from the conventional ingot-forming method to the continuous casting method. Coarse columnar crystals tend to form slowly in slabs manufactured by the continuous casting method9
Compared to ingot slabs produced through blooming and rolling, they are inferior in terms of ridging. In order to overcome this problem, 11ia stirring of the molten metal and low-temperature casting technology have been introduced, and the structure of continuously cast slabs has been considerably improved, but ridging has not yet been sufficiently reduced.

Rigging in 430 steel is thought to be due to the coarse cast structure formed during solidification being carried over to the hot-rolled steel strip as a band-like structure, which remains without collapsing during the subsequent cold rolling and annealing processes. It takes a lot of thought.

In this band-like structure, each band has a crystallographically different orientation, and the plastic anisotropy VC between the bands based on this causes a reching phenomenon.

Therefore, many of the proposed ridging reform measures for 450 steel up to this point are related to the refinement of the band-like structure of the hot-rolled steel strip.

Ordinary 450 steel contains austenite-forming impurity elements such as CZ, so at high temperatures it becomes a two-phase structure containing a part of austenite phase (γ phase) in addition to a ferrite phase (α phase). A method of randomizing the structure using this γ phase has been proposed in the past. - For example, hot-rolled steel strip is
There is a method of randomizing the structure by dispersing the γ phase that precipitates by reheating above the transformation point and transforming the γ phase during the subsequent cooling process. According to U.S. Pat. No. 2,851,384, the steel composition has the following formula: -stenite potential A A = 288 (-C) + 350 (CHN) + 22 (CHNI)
17.5 [%Ma] -18,75 (%Cr) -54
It is said that when the value of C%St)+338.1 is set to 651i or more, the effect of improving ridging is large.

However, the γ phase precipitated by reheating tends to re-precipitate at the location of the transformed phase (the phase generated by transformation from the prior γ phase) in the hot rolled steel strip, so this method does not destroy the band structure.FX Difficult.

Moreover, the process becomes complicated.

A method 410 that utilizes austenite potential without subjecting a hot rolled steel strip to special heat treatment as described above. That is, if we look at the relationship between austenite potential, hot rolling conditions, and ridging.

It has been shown that under the same hot rolling conditions, the higher the austenite potential, the more suppressed the occurrence of ridging. In addition, steels with high austenite potential have an increased amount of γ phase at high temperatures.

This is due to the fact that as the γ phase increases, the band-like structure of the (α+γ) phase becomes finer.

However, it has not been possible to reduce H-riding simply by increasing the austenite potential.

The seventh reason is that the concept of austenite potential (numerical value) was only devised as an indicator of the amount of γ-phase precipitation, and is not a property of the γ-phase.

(The amount of 11γ phase precipitated depends not only on the alloy element composition but also on the temperature; (2) The nose of the γ phase precipitation curve upon moxibustion IjAmh varies significantly depending on the alloy element composition.

It seems that this point has not been considered at all.

In this way, the conventional ridging modification using the γ phase
A major problem arises in the fact that the appearance of the RB and r phases is recognized as a function of only the austenite potential, and that only the amount of precipitation of 7 is utilized.

The present inventors conducted a detailed metallurgical study of the properties of the γ phase and the influence of alloy components on it, as well as the role of the γ phase during hot working, in relation to the occurrence of ridging in 450 steel. The present invention was based on the finding that medium ON has a large influence on the properties of the γ phase present in thermodynamics.

According to the present invention, n is Cr2 15.5% to 17.5'lG,
C: 0.01 S ~ 0.04 arrogant, N: 0.
04 S ~ O, O'8 A ferritic thread stainless steel is provided which contains F and unavoidably mixed impurities in the remainder.

In the steel of the present invention, N is effective in improving the metal structure during the hot rolling process of copper. That is, it is possible to facilitate the precipitation of the 1γ phase at the hot abrasive temperature and lower the nose temperature of the γ phase precipitation curve, thereby performing rolling in a two-phase state and refining the 9 structure. To achieve this effect, N
04% is necessary, and if it exceeds α08, the formability of the product steel sheet will be impaired.

Since C has the effect of increasing the nose temperature of the γ phase precipitation curve, in order to achieve the effect of N as described above, C must be 0.
It is necessary to limit it to 04 inches or less.

However, reducing the Ct to less than 0.01 cm will result in a large cost increase in steel manufacturing, so clhO101~
Limited to 004 cars.

From a comparison of Samples B3 and D2 in Examples described later, it is determined that the critical value of the lower limit amount of N is around α04. Moreover, the critical value of the upper limit amount of C from samples B5 and Dl is also α04-i! It is determined that it was after 11.

The steel of the present invention may contain impurities that are punched in the composition specified in JI8 as 8US450.

Adjustment line for N amount in manufacturing steel of the present invention 1) Method of blowing Ar, N, and gas alternately.

2) A method of adding 'fJ-modified Fs-Cr and stimulated Mn.

6) It can be easily carried out by using a combination of N, gas 2 and 211A methods.

Next, the present invention will be specifically explained using an example.

Copper with the composition shown in Dokishu 1, that is, actively adding N, C
The non-inventive steel that limits the
0 steel was bath-produced in a 4.0 ton electric arc furnace and refined by a converter-vacuum degassing method to produce a continuous cast slab with a thickness of 155M. Cut into 30m x 50m test pieces and heat treated (water cooling)
Then, the amount of γ phase was added. At this time, since Or@ transforms into a martensite phase by cooling at high temperatures, the amount of this martensite phase was regarded as the amount of γ phase. Measuring and illuminating characters -
It was carried out using the point counting method.

These slabs were heated to a temperature of 900° to 1500° C. for 2 hours and cooled with water. The amount of precipitated γ phase was measured, and the results are shown in the attached drawings. From this diagram.

As a structural difference between conventional steel and the steel of the present invention.

(1) In conventional steel D1, the nose temperature of the hr phase precipitation curve is 1
While it melts at 100°C, 1-inventive steel B1 melts at 950°~
Ruru to 1000℃.

(2) In the region from the nose temperature to the A1 transformation point of the larva, the amount of γ phase in conventional steel decreases rapidly as the temperature decreases, whereas in the steel of the present invention, the amount of γ phase decreases in the region below the nose temperature.
As a result of the phase reduction rate being 7, a considerable amount of the γ phase remains up to just above the transformation point of the copper.

The following points can be mentioned.

There is a big difference between the steel of the present invention and the conventional steel regarding the precipitation of the γ phase. The present inventors concluded that N is effective in improving the metal structure during the hot rolling process.

The usual heat treatment method for 430 steel is as follows.

2) Completion of rough rolling 980℃ 30m5) Finish rolling inlet 950℃ 4) Z exit 800℃ 56■5
) Winding #) 700 to 550°C Comparing the hot rolling conditions exemplified above in 1 above and the nose position of the γ phase precipitation curve shown in the drawing, it is found that the points of the metallographic structure during hot rolling in the steel of the present invention and the conventional steel. You can see that there is a big difference. As can be seen from the shape of the γ phase precipitation curve shown in Figure 1 (2) for the steel of the present invention, rough rolling and finishing can be performed in the two-phase region of (α + γ) in the rolling stage. to 0%,
It can be seen that at the hot geothermal stage of 10 sO' to 900°C, there is a two-phase region pressure abrasive that contains Iwaokata or a similar amount of γ phase. On the other hand, with conventional steel, the nose temperature [(approximately 1100
Because the amount of γ phase that disappears is large in the temperature range below ℃), there is very little γ phase remaining in the latter half of rough rolling and finish rolling. It can be seen that hot ground steel strips have been manufactured without achieving much success.

The main effects obtained by expanding the two-phase region compression to the low temperature side achieved by the present invention are as follows. First, (
In the two-phase region rolling of α + γ), due to the presence of the γ phase with extremely high hot strength, processing strain is transferred to the α phase with low strength and becomes an effective driving force for recrystallization of the α phase, and secondly, Since most of the γ phase transforms at a low temperature during the finishing rolling stage, it can be seen that the crystal grains are not randomized due to transformation.

Based on this knowledge, in normal hot rolling.

It can be seen that it is essential to contain HN in order to realize rolling in the 72 (α + γ) two-phase region, or that the effect of N on improving ridging is due to the effect of rolling in the 12-phase region as shown in 7. I'm here.

As described above, when using the lγ phase, conventionally only the amount of the Or phase was taken into consideration to increase the nine-austenite hotential.
It is clear that the method does not substantially achieve n*v.

In order to demonstrate the litching properties of the steel of the present invention, samples of ferritic stainless steel (450 steel) having the components shown in Table 2 were prepared in an electric arc furnace in the same manner as described above.

It was refined using a converter-vacuum degassing method to form a continuous slab of 155 m thick, heated and maintained at 1140° to 1160°C, and then rough rolled for 7 passes to form an intermediate plate thickness of 50 w1.

Furthermore, it is finished rolled with 6 tandem mill grinders.
．． 6 wx Jun hot ground copper strip, hot rolled steel strip 810'CX
After performing diffusion annealing for 6 hours, a cold-rolled and annealed plate of 0.7 m1I+ was finally produced according to a normal cold rolling process.

The occurrence of riching was determined using small tensile test pieces parallel to the rolling direction, after applying 2011 tensile deformations.
The FF value was determined by the center line average roughness obtained by adding tj# using a stylus roughness meter. Rigging was also visually determined. The riding properties and mechanical properties of the steel manufactured as described above.

The model formability is shown in Table 3.4.

It can be seen from Table 5 that the inventive steels B2 and B5 have significantly superior ridging properties compared to the comparative steels D2 and B3. Furthermore, from Table 1, it can be seen that the mechanical properties and formability of the steel of the present invention are equivalent to those of conventional steel.

*IRa: Center line average roughness (stylus roughness meter) Cut of
f ((i 4 wm Book 2 Relative evaluation of ridging tfli (visual judgment) t Best 2. Good 3. Average 4, Poor 5. Poor As described above, 2 Due to slight adjustments in the composition of the rL450m of the present invention, It is possible to greatly improve the durability, and its contribution to industry is enormous.

[Brief explanation of the drawing]

Attached drawing a is a diagram showing a comparison of the T-phase precipitation curves of the steel of the present invention and the conventional steel. Patent applicant Masahiro Matsui (2 others) 1 patent attorney representing Nisshin Steel Co., Ltd.
〃2 Procedural amendment written April 19, 1980 Haruki Shimada, Commissioner of the Japan Patent Office 1, Indication of the case Patent Application No. 195597 of 1982 3, Person making the amendment Relationship with the case Patent applicant 4, Agent 5 , Date of amendment order Voluntary action 6, No number of inventions increased by amendment 76 Detailed explanation of the invention column in the specification to be amended The Detailed explanation of the invention column in the specification is amended as follows. Ill QIJIil book 9jjjllIS line no r
40 tons Jir40 tons,” he corrected. (2) Correct "slab" t and "test piece j" in column 9, line 11 of the specification. (3) Correct Table 4 of item 151i of the specification as follows.

Claims (1)

[Claims] Cr: 15.5~t 7.5 S, C: 0.0
1-~0.041G. Ferritic stainless steel containing N: 0.04 to 0.08%, with the remainder consisting of 1) Fs and unavoidably mixed impurities.